This invention relates to a three dimensional display apparatus. More particularly, this invention relates to a three dimensional video display, such as a television, where a three dimensional moving image is seen without visual aids.
Display apparatus are in common use, e.g. in television receivers, computer interfaces, instrumentation and visual display units. Generally these are two dimensional display apparatus, in that they display only a two-dimensional picture.
A variety of cues are present in the picture provided by a two dimensional display, which provide the observer with a perception of depth. Such cues include shadow, size, perspective and the obscuring of far objects by close ones. However, two of the most important depth cues which are used by human vision are missing in a conventional two dimensional display. These are the difference between the views seen by the two eyes, and the continuous change of view which an observer would expect if he were moving his head with respect to a true three dimensional image. These two cues are necessary if the observer is to be given a proper perception of three dimensions.
Various display systems have been made which require the user to wear spectacles so that different views of a scene are presented to each eye. These provide human vision with stereoscopic information, but not with the information or cue expected from movement of the head. Further, such spectacles are inconvenient to wear.
Some attempts have been made to construct three dimensional display apparatus which do not require the user to wear spectacles. In many of these apparatus the picture seen on the screen depends on the direction from which the screen is viewed. The picture seen on the screen is that which would be seen of the three dimensional image from the same direction of observation if the three dimensional image were present in place of the display apparatus. The image presented by these apparatus provides all of the information needed by human vision to perceive depth, and so is apparently three dimensional. Most of these apparatus have moving parts which make them expensive and delicate.
For a conventional display such as a television screen, a picture of an object is assembled from a two dimensional array of pixels. In a similar way a three dimensional image of an object might be assembled from a three dimensional array of pixels. However, such an apparatus would be complicated and expensive. Moreover, opaque objects could not be properly displayed, as a front object would not block out the emitted light used to depict a rear object, and hence, the rear object would be seen through the front one as an apparition.
Holograms are known to be able in principle to reproduce an image of any three dimensional object. Holograms work by providing a surface which modulates light as a function of both position and angle. Modulation of light as a function of position is what is provided by an ordinary video display. It is the additional ability to modulate light as a function of angle which is required for a three dimensional display. Thus, logically, it might be desirable to provide a display device which displays holograms. However, the resolution required to provide a hologram is determined by the wavelength of light. As a result, the video mosaic (i.e. pixel array) required to display a hologram must be extremely fine, and for large area display devices such as televisions, the cost of providing such resolution is prohibitive.
Another manner of modulating light as a function of position and angle is proposed by Meacham, G. B. K., "Autostereoscopic Displays--Past and Future", Proc SPIE Int. Soc. Opt. Eng. Vol. 624 Advances in Display Technology VI, pp. 90-101, 1986. Meacham proposes putting a spatial light modulator in front of a video display with a movable vertical slit being provided in front of each pixel. If the pixel is also shaped as a vertical line, then the relative positions of the pixel and slit will determine the direction from which rays may leave the spatial light modulator. The disadvantages of such a system are the requirement of moving parts as well as the fact that the resolution of the video display must be very fine.